var secretWord = 'apple',
    letters    = '';

function play(letter) {
  if(letters.indexOf(letter) != -1) {
    console.log("You've already tried '" + letter + "' ...");
  }
  else {
    letters += letter;

    if(secretWord.indexOf(letter) == -1) {
      console.log(letter + ' -> wrong!');
    }
    else {
      var progress = secretWord.replace(new RegExp('[^' + letters + ']', 'g'), '_');

      console.log(letter + ' -> ' + progress);
      
      if(progress == secretWord) {
        console.log('Well done!');
      }
    }
  }
}

play('i');
play('e');
play('p');
play('e');
play('a');
play('l');